<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .item{
      width: 100px;
      height: 50px;
      border:1px solid skyblue;
      float: left;
      margin-right: 10px; 

    }
  </style>
</head>
<body>
  <div class="container">
    <h2 class='page-header'>点击切换颜色</h2>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
  </div>
  <script>
    let items = document.getElementsByClassName('item')
    for(let i=0;i<items.length;i++){
      items[i].onclick = function(){
        // this.style.backgroundColor = 'pink'
        items[i].style.backgroundColor = 'pink'
      }
    }
  </script>
</body>
</html>